1. Field of the Invention
The present invention relates to a switching power supply apparatus provided with an overload detecting circuit which is high in detection accuracy and simple in configuration.
2. Description of the Background Art
Current resonance type switching power supply apparatus represented by resonance type DC-DC converters or synchronous rectification boost type converters have been used as power supply apparatuses in various kinds of electronic equipment because they are highly efficient and suitable to be thinned. This kind of switching power supply apparatus is typically provided with a protective function by which an overload state of the switching power supply apparatus is detected and ON-driving of switching elements such as IGBTs (insulated-gate bipolar transistors) or MOS-FETs (metal-oxide-semiconductor field-effect transistor) are stopped, so that a load, the switching elements, etc. can be protected.
In the background art, the aforementioned detection of overload depends on monitoring the output power of the switching power supply apparatus, for example, on the secondary side of an isolation transformer thereof. However, the switching power supply apparatus provided with the overload detecting circuit on the secondary side of the isolation transformer has a large-scale configuration. In addition, it cannot be denied that the configuration causes increase in the cost of the switching power supply apparatus. Therefore, there has been heretofore proposed that a load state is monitored on the primary side of the isolation transformer, for example, as disclosed in U.S. Pat. No. 6,087,782 or Japanese Patent No. JP-A-2012-170218.
Specifically, U.S. Pat. No. 6,087,782 suggests that, in a DC-DC converter provided with switching elements which switch DC power to control a current flowing into an isolation transformer, effective electric power supplied to a load is obtained from an integrated value of the product of a VS voltage generated at one end of a primary winding in the isolation transformer and a resonance current flowing into the primary winding of the isolation transformer so that the load state can be detected based on the obtained effective electric power. On the other hand, JP-A-2012-170218 discloses a technique in which the load state is detected based on an average value of the resonance current flowing into the isolation transformer.
In the technique disclosed in U.S. Pat. No. 6,087,782, it cannot be denied that the circuit scale for computing the aforementioned effective electric power becomes large. Moreover, when the detection accuracy of the VS voltage and the resonance current and the calculation accuracy in an integrating circuit are taken into consideration, it is difficult to detect an overload state with high accuracy. In addition, in the technique disclosed in JP-A-2012-170218, there is a problem that the circuit scale becomes large because an averaging circuit, a multiplying circuit, etc. are necessary for averaging the resonance current.